I am using a large hard plastic cooler and a 40 watt bulb. Yours sounds bigger, I'd try a 50 or maybe 60 watt. I've heard that if the wattage is too high you get more temp variance.
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I would try 2 40 watt or 3 25 watt it is better to have more then 1 bulb just in case one burns out that and you can spread the heat out more evenly